Power supplies for many electronic devices employ a pulse width modulator. These power supplies, acting in a switch mode, either turn full on or full off and provide a stream of current pulses.
Many electronic devices also employ microprocessors or other digital circuits which require one or more clock signals for synchronization. For example, a clock signal permits the precise tuning of events in the microprocessor. Typical microprocessors may be synchronized by a free running oscillator, such as a crystal-driven circuit, an LC-tuned circuit, or an external clock source. Clock rates up to and beyond 40 megahertz are common in personal computers. The various parameters of a clock signal are typically specified for a microprocessor including frequency ranges.
Power supplies and high performance, microprocessor-based devices using leading edge, high-speed circuits are particularly susceptible to generating and radiating unwanted electromagnetic interference (EMI), which can interfere with other devices located in close proximity. The spectral components of the unwanted EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of the clock circuit.
Conventional techniques for reducing EMI emissions include either a large and expensive passive inductor capacitor filter, or a combination of a shielding technique provided by an enclosure and filtering components. In many cases, filtering and shielding can easily add several dollars of cost to a system, and may not be enough to allow a system to pass federal EMI regulations. Electronic devices must meet maximum EMI radiation limits as specified by federal regulations and comparable regulations in other countries. The federal regulations are designed to ensure that electronic devices do not interfere with each other. Recent federal requirements call for PC motherboards to be able to pass EMI emission tests in an “open box” configuration, so manufacturers are not able to rely on the shielding provided by an enclosure to meet EMI emission requirements.
Federal regulations are concerned with peak emissions of a device, such as a power supply, not average emissions. Thus, any techniques that can reduce the peak energy of a device will help the device meet federal requirements. Rather than concentrating or centralizing all unwanted EMI emissions at a single frequency, a spread spectrum technique is often utilized. In a spread spectrum technique, the EMI emissions are spread out or dispersed over a range of frequencies, instead of being concentrated at one particular frequency. The reduction in a devices peak EMI emission can be as great 10 dB through use of a spread spectrum technique. The same total amount of EMI emissions is still present; however, the peak value is reduced.
Prior art spread spectrum techniques utilize numerous electrical components including a crystal oscillator to provide the necessary frequency change. The type and number of necessary electrical components in prior art electrical devices significantly increase the cost of the overall electrical device. In addition, these prior art techniques suffer from poor jitter performance.
In conjunction with a power supply, pulse width modulators are used in conjunction with power supplies as a switching device which either turns the power supply full on or full off. With the repetitive pulsating currents of the power supply, peak EMI emissions are produced at a fundamental frequency of the power supply. It is desirous to spread or disperse these unwanted peak EMI emissions over a range of frequencies.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for an inexpensive spread spectrum design for electronic devices such as power supplies which will utilize few inexpensive components, while still provide the necessary change in frequency to reduce peak EMI emissions.